In electrochemical processes at solid-liquid interfaces, one of the main factors controlling mass transfer is the formation of boundary layers. These layers have a thickness between 0.15 and 0.7 mm, depending on the flow velocity of the liquid. Classical electrochemical techniques only give overall measurements of mass transfer; the aim of this study was to measure local mass transfer using holographic interferometry in real time, a technique particularly well adapted to the use of cells with complex forms. Two processes were observed: electrolysis and electrodialysis. The refractive index profile in the boundary layer was observed as a function of the liquid flow-rate and the applied difference in potential. Because of high refractive-index gradients in the boundary layer, the observed profile may be considerably distorted by light-deflection effects. Calculations have shown, however, that light-deflection errors may be kept within tolerable limits by using sufficiently dilute solutions. These solutions of limited concentration still allow useful results to be obtained.